Metal oxide varistors (MOVs) are common electrical components typically used to protect electrical circuits and equipment from high voltage transients. MOV's are highly non-linear devices whose characteristics result from the double Schottky barrier formed across the grain boundaries formed during the sintering process. The polycrystalline structure is primarily zinc oxide, but also has small additions of Bi2O3, Sb2O3, SiO2 and other oxide constituents. The number of grain boundaries between conductive plates and the cocktail of oxides used in the formulation of the MOV determine the threshold at which an MOV begins to conduct. MOV's are placed in parallel with the systems to be protected and are therefore subject to constant electrical stress.
Further, MOV's are subjected to periodic transient voltages and overvoltage conditions which apply further electrical stress. As a result of these stresses MOV's tend to degrade over time resulting in higher leakage current. At the end of their electrical lives, MOV's tend to fail catastrophically. End-of-life failures come in various forms. Failure due to fragmentation caused by excessive transient voltage is one type of end-of-life failure. Another failure type is thermal runaway caused by either degradation of the MOV and/or a sustained abnormal overvoltage condition. A thermal disconnect is used to open the device in the event of sustained overvoltage or thermal runaway due in part to the aforementioned electrical stresses noted above. It is desirable to have the thermal disconnect mechanism in very close proximity to the MOV disk so that thermal response time is as fast as possible. Therefore the purpose of a thermal disconnect MOV is to provide for relatively benign failure when subjected to conditions leading to thermal runaway.
Although thermally protected varistors are presently available, the currently available thermal disconnect varistors comprise complicated assemblies and are costly to manufacture. A drawback of known approaches of thermally protected varistors is that they are one-time use components that must be replaced once the thermal disconnect has been triggered. As the thermal disconnect is typically enclosed in a casing, an individual maintaining the equipment may be unable to easily determine when the thermal disconnect has been triggered and needs to be replaced.
Thus, there presently exists a need for an efficiently-constructed varistor for protecting sensitive electrical circuits and equipment from abnormal overvoltage transients that can be easily maintained and serviced.